1. Field of the Invention
The present invention relates to an apparatus for producing an optical fiber preform, more particularly relates to an optical fiber soot synthesis apparatus which synthesizes with a precise quality an optical fiber soot formed around a fiber starting member forming the core of the optical fiber.
2. Description of the Related Art
An optical fibers is comprised of a core of silica based glass and a cladding of silica based glass formed around the core and with an index of refraction lower than the core.
When producing an optical fiber, first the portion forming the core is produced by a vapor phase axial deposition (VAD) method and glassified to form the fiber starting member. Next, an optical fiber soot forming the cladding is synthesized around the fiber starting member. The optical fiber soot is synthesized by supplying, for example, SiCl.sub.4 gas as the material of the optical fiber soot to the soot synthesis burner and hydrogen and oxygen as the material for the oxyhydrogen flame while controlling the mass so as to cause a chemical vapor deposition (CVD) reaction. This enables the production of an optical fiber preform comprised of a fiber starting member around which is formed an optical fiber soot.
Then, the optical fiber preform is heated and drawn. For example, in the case of a single mode optical fiber, the optical fiber is formed with a core of a diameter of 10 .mu.m and a cladding of an outer diameter of 125 .mu.m. This optical fiber is covered with a plastic resin layer.
The ratio between the core diameter and the cladding diameter must be precisely defined, so it is necessary to precisely produce the optical fiber soot on the fiber starting member.
In the past, the method was adopted of stopping the synthesis in the middle of the step of synthesizing the optical fiber soot and measuring the overall weight. The weight of the fiber starting member is known in advance, so by subtracting the weight of the fiber starting member from the overall weight, the weight of the optical fiber soot could be determined and the weight of the optical fiber soot could then be found.
Another conventional method was to glassify the optical fiber soot in the glassification process drawing the fiber from the optical fiber preform, then measure the ratio between the core diameter and the cladding diameter by optical measurement and if the cladding layer was insufficient, to resynthesize the optical fiber soot, and if excessive to etch it away.
When resynthesizing in the case of an insufficient cladding layer, the above-mentioned methods all present problems in that the once cooled cladding layer is reheated, so not only does the productivity fall, but the cladding layer also is placed in a state where it is easily damaged by heat shock.
Further, excessive cladding layer is etched away, so the productivity declines.
Therefore, attempts have been made to strictly control the mass, manage the manufacturing time, etc. so as to produce an optical fiber soot neither insufficient nor excessive, but fluctuations in the control of the mass are unavoidable due to limitations in the range of control of the mass control valves and thus precise mass control is not realized. As a result, even if the time is strictly managed, it is difficult to produce an optical fiber having a precise and stable core-cladding diameter ratio.
As disclosed in Japanese Unexamined Published Patent Application (Kokai) No. 63(1988)-144139, Japanese Unexamined Published Patent Application (Kokai) No. 63(1988)-285130, Japanese Unexamined Published Patent Application (Kokai) No. 63(1988)-285131, and Japanese Unexamined Published Patent Application (Kokai) No. 2(1990)-167838, attempts have been made to solve this problem by measuring the weight of the optical fiber soot in real time of the production process of the optical fiber.
The apparatus for production of an optical fiber preform disclosed in Japanese Unexamined Published Patent Application (Kokai) No. 63-144139 is a horizontal type apparatus for producing an optical fiber preform which is comprised of a base, two brackets standing at the both sides of the base, and a motor mounted to these brackets, the motor turning the fiber starting member in the horizontal direction. Under the base is arranged a load cell, which load cell measures the total weight of the overall structure of the base and brackets. The weight of the apparatus as a whole is usually about several hundred kilograms, so it is necessary to use a load cell with a wide measurement range. Due to restrictions in the resolution and precision of the load cell, it is not possible to measure the weight of the optical fiber soot to a precision of about 100 g.
In the optical fiber soot synthesis apparatus disclosed in Japanese Unexamined Published Patent Application (Kokai) No. 63-285130, there is disclosed a vertical type apparatus for producing an optical fiber preform wherein the fiber starting member is suspended vertically and rotated about the vertical direction axis and a burner is raised and lowered near the fiber starting member so as to synthesize an optical fiber soot on the fiber starting member. This apparatus for synthesizing an optical fiber soot first of all suffers from a disadvantage in that due to the vertical construction, the optical fiber soot is not uniformly synthesized due to the effects of gravity. Further, it is structured so that the weight of the suspended fiber starting member and optical fiber soot is measured by a weight measuring apparatus attached to the top portion, but this is affected by the shaft for rotating the fiber starting member, the bearings, and the belt for transmitting the power to the rotational shaft, etc., and therefore the weight of the fiber starting member and the optical fiber soot cannot be measured accurately.
The apparatus for producing an optical fiber preform disclosed in Japanese Unexamined Published Patent Application (Kokai) No. 63-285131 solves the disadvantage in the apparatus for producing an optical fiber preform disclosed in the above-mentioned Japanese Unexamined Published Patent Application (Kokai) No. 63-144139 by providing an integral structure of balance weights horizontally axially supported at the top of the brackets and a fiber starting member rotating mechanism and further by providing under the fiber starting member rotating mechanism an actuator for detecting the rotation of the fiber starting member rotating mechanism. That is, in this apparatus for producing an optical fiber preform, the intent is to measure the weight of only the fiber starting member and the optical fiber soot by the balance weights. However, the measurement of weight in this apparatus for producing an optical fiber preform calculates the weight of the optical fiber soot by detecting the rotational movement of the balance weights and the fiber starting member rotating mechanism rotating about the upper support points of the brackets, so accurate calculation of the weight is not possible. Further, it is necessary to change the angle of the burner with respect to the fiber starting member in accordance with the above rotation, so the control and the mechanism become extremely complicated.
The apparatus for producing an optical fiber preform disclosed in Japanese Unexamined Published Patent Application (Kokai) No. 2-167838 makes use of a weight measuring apparatus of the fiber starting member, the optical fiber soot, and the mechanism for rotating and supporting the two utilizing the lever principle to measure the weight of the fiber starting member and the optical fiber soot. Even in this apparatus for producing an optical fiber preform, however, the overall weight of the rotational drive mechanism is detected by the weight measuring apparatus, so it is not possible to measure the weight with a high resolution. Further, the weight measuring apparatus disclosed in Japanese Unexamined Published Patent Application (Kokai) No. 2-167838 has a complicated structure using the lever principle so has the problems of large dimensions, high price, and complicated adjustment work.
Further, the weight of the optical fiber soot cannot be accurately found using just the weight detected by a soot weight detection sensor. It is necessary to consider the center of gravity position of the rotating optical fiber soot and the like. None of the above-mentioned prior art suggest a method for measuring the weight of an optical fiber soot precisely.